Non-Native Animal Species Control
To minimize and mitigate the impacts of low flows, the City of the New Braunfels will conduct non-native animal species control on an annual basis. Initial control efforts will be intense. Control of non-natives will include annual maintenance and monitoring and non-natives will be disposed of out of the floodplain. The non-native animal species that will be addressed include the suckermouth catfish, tilapia, nutria, and ramshorn snail.
Studies have shown that many fishes (especially small fish) have very similar food habitats. If non-native species are added to the aquatic ecosystems, greater competition or overlap among species is possible as these non-native species may be able to acquire resources with greater efficiency than native species. Suckermouth catfishes (Loricariidae) are a non-native fish species that has become established in the waters of Texas including the Comal River. Suckermouth catfishes prefer to feed on periphyton and algae. The fountain darter lays eggs on algae and loss of spawning habitat and possibly egg predation are potential threats from suckermouth catfish. There is some concern that excessive numbers of suckermouth catfishes could cause direct (potential displacement) and indirect effects (disruption of food supply) to the fountain darter in the Comal River. Suckermouth catfishes also burrow into the river banks, destabilizing them and causing the introduction of additional sediment load into the habitats.
Tilapia is another non-native fish species that can impact fountain darter habitat. Tilapia destroys vegetation by making bare ground nests. During times of low flow and drought this could further reduce already limited habitat for the fountain darter. Tilapia is a tropical species that will congregate in winter near spring openings and other warm water sources. When tilapia congregate this creates the opportunity to use seines, gill nets, cast nets or other methods to remove large quantities with minimal impact to the habitat. Artificial heating could be one method used to congregate fish in areas away from springs and endangered species to minimize the impacts from collection efforts. Similarly, tilapia tend to congregate in backwater pools during summer months. This may afford another opportunity for effective removal of the fish.
A non-native gastropod (giant ramshorn snail [Marisa cornuarietis]) also poses a threat to the Comal Springs ecosystem. The giant ramshorn snail, a species in the aquarium trade was first discovered in Landa Lake in 1984. This snail grazes on aquatic plants and in the 1990s played a major role in reducing plant biomass in Landa Lake. This snail prefers clear streams and pools with temperatures of at least 68 degrees. When exposed to lower temperatures, the snails withdraw into their shells and only survive. The population has diminished since the mid-1990s, the potential for future alteration of plant communities by the Ramshorn in the Comal ecosystems remains. The strong preference of fountain darters for aquatic vegetation highlights the concern posed by the grazing activities of the giant ramshorn snail. This species will be monitored closely so that it does not significantly reduce the available fountain darter habitat.